1. Field of the Invention
The present invention generally relates to an automatic multiplex data communication system for transmitting operation data about a large quantity of operation members such as head lamps via a small number of transmission lines to a main control unit. More specifically, the present invention is directed to an automobile multiplex data communication system including a low-cost fail-safe function with respect to fails occurring in a LAN communication network.
2. Description of the Prior Art
Various electronic components and signal processing methods have been developed for automobiles. To electrically communicate a large number of electronic terminals (components, sensors, displays) with a main control unit, a large quantity of wire harness iS required. However, such a wire harness may cause problems of heavy weights and deterioration of system reliabilities.
To eliminate heavy wire harness, LAN (local area network) communication systems are utilized in automobile communication systems.
Very recently, a so-called "multiplex data communication system" has been gradually utilized in certain automobiles, in which various operation data about a large number of operation members such as brake lamps and power-window switches are transmitted via a small number of multiplex transmission line between a main (parent) station and child stations with such operation members. In the multiplex data communication system, generally speaking, the operation information or data of the operation members, e.g., switches connected to these child stations, is transmitted between the child stations and the parent station. Accordingly, the parent station can detect the operation conditions of these respective operation members based upon the operation information. Then, upon detection of changes in the operation conditions, the parent station sends drive information or data via the multiplex transmission line to the relevant child station to which the terminal unit is connected, so that this terminal unit can be driven under control of the parent station. When the above-described operation information about various operation units is transmitted/received to/from the parent station, if the signal communication lines (LAN) would be disconnected, or the signals would be adversely influenced by receiving electromagnetic noises, such a communication error (erroneous information) is detectable by performing the parity bit check, so that the fail-safe function of this system can be done.
As to the conventional fail-safe function, a more detailed described will now be explained with reference to FIG. 1. FIG. 1 represents a major circuit arrangement of the conventional automobile communication system equipped with the fail-safe function.
In the conventional automobile communication system, a fuse box 1150 is arranged by a fuse "FS" and a switch "SW". A voltage of a power source (battery, not shown in detail) is applied via this fuse "FS" and a power source line "LPA" to a child station 1100 and also via another power source line "LPB" to a communication unit 1401. This power source line LPA is also used to connect this child station 1100 with a parent station (not shown) and many other child stations. Since this switch SW is of a normally-close type switch, the power supply voltage is normally applied to the communication units of the plural child stations. The communication unit 1401 is constructed of a signal receiving unit 1410, an inverter circuit 1416, an OR gate circuit 1417, an AND gate circuit 1418, and a drive unit 1420.
In this automobile communication system shown in FIG. 1, for instance, when one power source line "LPB" is disconnected, and then the parity bit becomes an even bit, it can be judged that a so-called "communication error" happens to occur. As a result, a logic value "1" is outputted from an error terminal "ERR" of the signal receiving unit 1410. This error logic signal having the signal level of "1" is supplied via the OR gate circuit 1417 to an input terminal "A1" of the drive unit 1420. Thus, a logic signal having a level of "1" is outputted from an output terminal "B1" of the drive unit 1420, thereby causing a headlamp 1403 to be forcibly turned ON. (lightening).
As described above, in case that such a communication error happens to occur, the preselected terminal unit such as the headlamp 1403 is compulsorily turned ON and this compulsory turn-ON condition is continued. Accordingly, for instance, when driving operation is accomplished, such a compulsory turn-ON state must be released, namely the headlamp 1403 must be turned OFF. In this case, since a car driver manipulates the switch "SW" of the fuse box 1150 to cut out the power supply from the battery to the communication unit 1401 of this child station, the headlamp 1403 which has been compulsorily turned ON, can be turned OFF.
As described above, in the conventional automobile multiplex data communications system, the specific switch "SW" must be employed to turn OFF the headlamp 1403 which has been forcibly turned ON during the occurrence of such a communication error. This may cause high manufacturing cost of this communication system. Moreover, every car driver must manipulate such a specific switch provided in the fuse box 1150 only when the communication errors happen to occur. A cumbersome switch operation is required.
On the other hand, according to another conventional communication system, when a communication error happens to occur, such a control arrangement that the supply of power to the communication unit is controlled by either the parent station, or the relevant children station, has been proposed.
However, in the latter conventional automobile multiplex data communication system, since all of the child stations are wholely power-controlled by the parent station, or the relevant children station, there are some risks that the respective terminal units cannot be separately controlled.